The present invention relates to a filter medium in the form of a stable porous body of granules of a fire-resistant material bonded together.
From U.S. Pat. No. 3,524,548 there is known a rigid porous filter for filtration of molten aluminum, which consists of a first granulate-like fire-resistant material, which is not attacked by molten aluminum, and which has a binder a glass-like material, which contains not more than 10% silicates.
As granulate there is mentioned "fused alumina" or "tabular alumina". With "fused" or "tabular alumina" one is dealing with fused corundum broken into pieces. This material produces a filter with relatively slight permeability and porosity. The filter effectiveness and filtering capacity is restricted by the internal structure. For this reason in practice bundles of filter tubes are normally installed, in order to achieve the desired amounts of flow.
It is also known from German OS No. 22 27 029 that such kinds of rigid filter elements, for example in the form of tubes, are very fragile.
It can be assumed that this fragility has its basis at least partially in the fact that in the firing process unavoidable stresses and consequential points of fracture arise. Additionally disadvantageous is the high weight of the filter elements according to U.S. Pat. No. 3,524,548 and the long preparatory heating up time thereby caused, before the molten aluminum can be directed through the filter element.
For the start of the filtration and also during the filtration relatively large pressure differences must prevail, in order to drive the molten aluminum through the filter element.
By means of a filter element of an entirely different kind an attempt has been made to eliminate the disadvantages, such as great pressure differences during filtration and restricted filtering capacity. In Swiss Pat. specification No. 622 230 a filter element is described, which is manufactured by impregnation of a polyurethane foam with a ceramic suspension, pressing out of the excess suspension, drying and firing. According to this method one obtains an approximate replica of the original organic foam in rigid ceramic form. Filter elements of this kind have a high filtering capacity and high rates of through flow, and thus enable themselves to be employed in the form of simple filter plates. There is inherent in these filter elements the disadvantage that they are expensive in manufacture.
U.S. Pat. No. 4,278,544 discloses a filter medium for fluid, which is a sintered body in which alumina refractory is 100 parts by weight, more than 95% by weight of said refractory being pelletized spheroids of less than 1.0 mm in particle size, while inorganic binder having particle size less than 40 microns is in the range 15-30 parts by weight and fluoride and/or oxide of lithium is in the range 0.1-1 part by weight, and in which filter medium the mean pore diameter is in the range 500-1 microns and the porosity is in the range from 15 to 40%. The refractory material is selected from the following materials: alumina, corundum, mullite, bauxite, diaspora and sillimanite. Filter plates manufactured in accordance with the U.S. Pat. No. 4,278,544 suffer from a number of disadvantages. Firstly, the respondant filter plates would be extremely heavy, for example, a 17".times.17".times.1" plate would weigh about 11,698 grams as compared to about 3878 grams for a similarly sized filter in accordance with the present invention. Thus, the filter plate would be difficult to handle. Secondly, and more important, filter plates of the size set forth above would fracture under the weight of the metallostatic head is used to filter molten metal.
The object of the present invention is to overcome the disadvantages mentioned and to provide a filter plate, which can be manufactured easily and in consistent quality, that has a good filtering efficiency, is well wetted by the material to be filtered, possesses a high filtered capacity and is easy to use.